(a) Technical Field
The present invention relates to a technique for localizing a sound source. More particularly, it relates to a technique for localizing a sound source, which can estimate sound pressure distribution with respect to a low frequency noise source, and can improve the resolution of sound field visualization with respect to the low frequency noise source.
(b) Background Art
As the noise characteristics of various products such as vehicles and home appliances are recognized as an important performance indicator, efforts to manufacture products producing less noise have been made at the development stage. A method of finding the location and the cause of noise generation and reducing noise through design modification is being used to reduce noise of products as well. For this, a measurement technique which finds the location of a noise source is primarily needed.
Recently, a method of measuring the intensity of a sound pressure using an intensity probe and a method of localizing a sound source using a microphone array beam forming method have been widely used.
A method of localizing a sound source using a spherical microphone array, which has been relatively recently developed, uses a microphone that includes a plurality of noise-measuring sensors, and utilizes a technique of calculating the intensity distribution of a noise source through signal processing using the phase difference of measured signals according to a distance difference between the noise source and the noise-measuring sensor and localizing the location of the noise source according to the intensity of the noise source. The measurement accuracy of the beam forming method is determined by the number of the sensors used. Generally, it is known that more sensors improve its performance.
On the other hand, a method for measuring the location of a noise source and visualizing a sound field in an interior space of a vehicle includes estimating the location of a noise source via a beam forming method, acquiring an omnidirectional (360 degrees) image of the interior space of the vehicle where the noise source exists using a plurality (e.g., twelve) of image sensors or image capturing devices (e.g., cameras), and displaying the location of the noise source obtained by the beam forming method on the omnidirectional image acquired by the image sensors.
A spherical microphone array used for localization of a noise source is disclosed in Korean Patent Application No. 2011-0093086, filed on Sep. 15, 2011 by the present applicant and inventor and is hereby incorporated by reference in its entirety.
FIG. 1 is a view illustrating a typical beam forming method using a spherical microphone array. For localization of a noise source using a spherical microphone array sensor 10, a plurality of microphones 12 are fixedly arranged on the surface of a spherical body 11. In the beam forming method, beam power is obtained to localize a sound source. The location of an actual sound source 21 is estimated by calculating a time delay between the location of each sound source candidate and each microphone 12 on the surface of the spherical body 11. In this case, an interrelation (beam power) between the sound pressure signals of each microphone 12 and the sound pressure signals at the locations of each sound source candidate is calculated, and the highest location is estimated to be the location of the actual sound source 21.
In the typical beam forming method, beam power needs to be calculated, and the location of a noise source is estimated by controlling the time delay between sound pressure signals received by each microphone. The beam power can be expressed as Equation (1) below.
                              z          (                                    k              →                        ,            t                    )                =                              ∑                          m              =              1                        M                    ⁢                                    p              m                        (                          t              -                                                Δ                  m                                ⁡                                  (                                      k                    →                                    )                                                      )                                              (        1        )            
where z({right arrow over (k)},t) is a beam power, m and M denote a microphone index and the total number of microphones, respectively, Δm({right arrow over (k)})={right arrow over (k)}·{right arrow over (r)}m/c is a time delay, {right arrow over (k)} is a traveling direction of a sound wave, {right arrow over (r)}m is a distance from a reference point to m-th microphone, and c is a propagation velocity of a sound wave in the air.
However, in the above delay-sum beam forming technique, there is a limitation in localization of a low frequency noise source (i.e., equal to or less than about 500 Hz) that has a sufficiently long wavelength compared to an interval between microphones adjacent to each other in the microphone array. This limitation inevitably occurs in the delay-sum beam forming technique. In the case of a low frequency noise source, when a typical beam forming method is used, the resolution is reduced due to a reduction of visualization performance, causing a limitation in the localization of the noise source.
FIG. 2 is a view illustrating a limitation of a typical method. When a spherical microphone array having a specific radius a is used, as shown in FIG. 2, the localization of a noise source is difficult at a low frequency band (e.g., a low ka region equal to or less than about 500 Hz).
Accordingly, an acoustic holography technique may be used to overcome the above limitation. Also known is a commercialized technology (acoustic holography technology for a spherical microphone array) that uses a spherical harmonic function in sound field visualization for a spherical microphone array). In the acoustic holography technique, the sound field estimation is performed using a spherical harmonic function when r≧a. However, in the above acoustic holography technique, only near field estimation is possible due to a limitation on the numerical formula. When a typical acoustic holography is used, frequent measurement (e.g., more than 15 times in the case of the indoor of a vehicle) is needed for the sound field visualization. Accordingly, a lot of numerical calculations are needed, and therefore analysis is delayed.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.